U.S. Pat. No. 4,580,568, Gianturco discloses a stainless steel stent made of zigzag loops of stainless steel wire. Such stents have come to be known as “Z-stents”. The delivery system for the Z-stent comprises a catheter with a tapered tip, fitted within a sheath. The sheath and catheter are advanced as a unit into the vascular system until the distal end of the assembly extends across the target site for stenting. Then, the taper tip catheter is withdrawn to leave the sheath in place, with its distal end extending across the stenting site. Then, an adaptor is fitted to the proximal end of the sheath to enable a compressed Z-stent to be advanced from the adaptor into the proximal end of the sheath. Then, the adaptor is removed and a polyethylene tube with a flat leading end is introduced into the proximal end of the sheath so that the leading end of the tube is in abutment with the proximal end of the Z-stent. By pushing on the tube, the Z-stent can be driven distally along the full length of the sheath until the stent resides, still within the sheath, but immediately adjacent the distal end of the sheath, and spanning the stenting site. At this point, by holding the tube against axial movement, and withdrawing the sheath proximally, the stent can be released into deployment at the stenting site, progressively, from its distal end, as the distal end of the sheath withdraws proximally through the stenting site, gradually to release the length of the stent, starting with the distal end of the length of the stent.
As the sheath withdraws proximally along the length of the stent, the stent radially expands away from the long axis of its delivery system. Because there is no structure of the delivery system within the stent envelope, so withdrawal of the delivery system, proximally away from the site of stenting, can be effected without any risk of disturbance of the stented tissue.
Above-mentioned U.S. Pat. No. 4,580,568 is a disclosure which occurred very early in the development of stent delivery systems, being based on a patent application filed in 1984. Over the last fifteen years there has been intensive development of delivery systems for self-expanding stents. For a relatively recent disclosure, reference is made to EP-A-819 411, with a priority date of 1996. The delivery system disclosed in this document confines the self-expanding stent between a bed on an inner tube and a sleeve surface on an outer tube, release of the stent being effected by proximal withdrawal of the outer sleeve. The drawings show the distal end of the delivery system abrupt and flat. In contrast, the document shows as prior art a schematic diagram of a delivery system in which a self-expanding stent is also confined in a bed between an inner tube and an outer sleeve, but the inner tube extends into an end zone, distal of the distal end of the stent, which is frusto-conical or tapered so as to provide the delivery system overall with a distal end zone which is tapered radially inwardly to its ultimate distal end face.
In both of these systems described in EP-A-819 411, the event of deployment of the stent is followed by proximal withdrawal, from within the stent envelope, of the inner tube. In both of the systems, the inner tube component of the delivery system, inside the stent envelope, has re-entrant surfaces associated with the bed in which the stent was originally confined. The present inventors have appreciated that such re-entrant surfaces are undesirable. Stents are characterised by a lattice structure in which the lattice-work openings get bigger as the radius of the stent expands during deployment. It is not inconceivable that bodily tissue within the wall of the lumen at the stenting site can protrude through lattice-work openings of the stent, into the bodily lumen, during or immediately following deployment of the stent. Then, when the inner tube is withdrawn proximally through the stented zone of the bodily lumen, any re-entrant surfaces on the inner tube distal of the proximal end of the stented zone could conceivably engage with bodily tissue inside the envelope defined by the deployed stent lattice, and this engagement could part the engaged bodily tissue from the lumen wall. Particles of tissue, once detached, could be carried away in the flow of bodily fluid within the lumen, conceivably with adverse or even fatal consequences. Therefore, the present inventors have concluded, any such re-entrant surfaces should be avoided, if at all possible.
For a recent disclosure of a stent delivery system which has no stent bed inside the stent envelope see U.S. Pat. No. 5,833,694 published Nov. 10, 1998, in which the FIG. 20 embodiment discloses a variation in which the delivery catheter has a uniform diameter and within it a pusher tube 22, the distal end 190 of which serves as a stop for the proximal end of the stent. To deploy the stent, the sheath is pulled back proximally while the distal end of the inner tube prevents proximal movement of the stent itself. A distinctive feature of the disclosure of U.S. Pat. No. 5,833,694, which distinguishes it from U.S. Pat. No 4,580,568, is its proposal to place a plurality of stent rings, one after another, by progressive proximal withdrawal steps of the sheath to release the stent rings one by one, at desired locations within the bodily lumen.
Another prior art disclosure of a stent delivery system is found in U.S. Pat. No. 5,782,855 Lau et al., in which a stent lies radially between an outer sheath and a balloon catheter. On the distal tip of the balloon catheter is a cone, into which is tucked a tapered distal tip of the outer sheath. For deployment of the stent, the sheath is withdrawn proximally with respect to the stent. After balloon expansion of the stent, the balloon catheter is withdrawn proximally so that the cone passes in the proximal direction the full length of the stent lumen. The cone has an exposed proximal-facing rim edge as is passes through the stent lumen.
U.S. Pat. No. 6,019,778 Cordis Corporation discloses a delivery apparatus for a self-expanding shape memory alloy stent which features a stent bed on an inner shaft and an outer sheath which includes a braided reinforcing layer. There is a stop on the shaft member, proximal of the stent bed, to prevent proximal movement of the stent when the outer sheath is withdrawn proximally to release the stent. The braided reinforcement layer is preferably made from stainless steel and is said to resist a tendency of the stent to become imbedded within the sheath which surrounds it.
EP-A-720 837 Fischell discloses an integrated double-function catheter system for balloon angioplasty and stent delivery. An outer sheath with a conically-shaped distal tip portion surrounds a stent. Radially inside the stent is a balloon catheter. The balloon is located well distal of the stent so as to allow better trackability of the distal end of the catheter over a flexible guidewire and through tortuous coronary arteries and through a long tight stenosis. The provision of the conically-shaped distal portion of the outer sheath is said to enable proper placement of the stent, even in cases of severe intimal dissection which could cause an intimal flap that could block the passage of an outer sheath having a blunt end.
EP-A-554 579 discloses a stent delivery device with coaxial shaft and sheath for a self-expanding stent. The sheath is provided at its distal tip with a protective tip which is bonded to the sheath thermally or with adhesive, or can be made integral with the sheath. This tip is said to reduce the likelihood of injury to the bodily lumen wall during advancement of the catheter in the lumen.
EP-A-119 688 Balco published September 1984 discloses a process and apparatus for restoring patency to bodily vessels in which a shape memory alloy wire is contained within an outer sheath and is abutted at its proximal end by a pushing shaft. It is deployed by withdrawing the sheath proximally. The diameter of the sheath surrounding the prosthesis is very much greater than the diameter of the sheath for the remainder of its transluminal length, over which it is a relatively snug fit with the pushing shaft. The sheath is said to be inserted, as by conventional techniques, into the aorta of the patient, in order that the prosthesis can be placed at an aneurysm.
U.S. Pat. No. 4,665,918 is another example of disclosure of a delivery system for a self-expanding stent held within a surrounding sleeve which is proximally withdrawn relative to a stent bed in a coaxial inner shaft, and with a tapered tip zone on the shaft which protrudes beyond the distal end of the surrounding sleeve.
U.S. Pat. No. 5,662,703 discloses a delivery device for a self-expanding stent, having an outer catheter surrounding an inner catheter and a tubular stent-retaining sheath formed of a rolling membrane. The self-expanding stent is located at the distal ends of the inner and outer catheters. The stent is radially inwardly constrained by a double-walled rollable membrane. The separate proximal ends of the radially inner and outer membrane portions are fixed respectively to inner and outer catheter components whereas the contiguous-distal ends of the membrane portions converge and narrow thereby to form a tapered tip. For stent release, the outer catheter is moved proximally at least twice the length of the stent in order to pull back proximally both the inner and outer layers of the membrane, thereby releasing the stent.
U.S. Pat. No. 5,735,859 discloses a stent delivery device comprising an inner and outer catheter and a stent covered by a thin-walled sheath. The inner catheter projects beyond the distal end of is fixed to the distal end of the outer catheter. The distal end of the sheath is releasably received in the distal section of the inner catheter distal to the stent. The sheath can be released from the distal section of the inner catheter and pulled back from the stent, thereby releasing said stent. The stent can either be self-expandable or expanded by a balloon. Where the distal end of the sheath is received in the distal section of the inner catheter, a step in the radially outside surface of the inner catheter is present.
EP-A-747 022 discloses a coil-reinforced retractable sleeve for a stent delivery catheter. One embodiment of the sleeve has a distal tip which tapers inwardly and is provided with a plurality of slits which extend proximally from the distal end of the sleeve and substantially parallel to the longitudinal axis of the sleeve, the slits functioning to provide the sleeve with a low profile adapted for traveling through a blood vessel.
EP-A-948 946 discloses apparatus and methods for deployment and release of an intraluminal graft for treating a stenosis, the graft being surrounded by a cylindrical cover which is withdrawn proximally to release the graft. The cover can have an atraumatic distal end of reduced diameter in which there are slits extending axially from the distal end wall.
WO 99/49929 discloses a rapid exchange delivery system for stenting a body lumen, with the stent being covered by a retractable sheath, and the stent itself being mounted on a balloon. In the drawings, it appears that the diameter of the sheath is somewhat greater radially outside the stent than in a distal end zone of the sheath, distal of the stent, touching the underlying balloon.
EP-A-850 655 discloses a catheter tip mold and cut process in which the molding process creates flash which extends beyond the desired catheter tip, which flash is then parted from the distal end of the molded catheter tip by use of a cutter. U.S. Pat. No. 5,843,090 is another disclosure of an inner catheter with a step at its distal end (see FIG. 6) when the outer catheter is withdrawn proximally. See also U.S. Pat. No. 5,743,874 for a further disclosure of an inner catheter with a step (FIG. 1, feature 81) in its outer surface.
Incorporated by reference are all the disclosures of all the above mentioned prior publications.